Demersal otter trawls

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Background - 31 March, 2014

A type of bottom trawl that has two rectangular 'doors' or 'otterboards' to keep the mouth of the funnel-shaped net open horizontally while the net is being towed. A vertical opening is maintained by weights on the bottom and floats on the top.

Target

A wide variety of benthic and demersal species (those that spend most of their lives on or near the seabed), including cod, haddock, hake and flatfish such as plaice and sole.

How they work

A type of bottom trawl that has two rectangular 'doors' or 'otterboards' to keep the mouth of the funnel-shaped net open horizontally while the net is being towed. A vertical opening is maintained by weights on the bottom and floats on the top. The lower edge of the net opening is dragged along the seabed with the aid of bobbins, rollers and 'rockhoppers', which can roll across or dig into the bottom, or bounce over obstacles.

Problems

Bottom trawling and dredging are both destructive and wasteful - seabed ecosystems are ploughed up and a wide range of organisms are crushed in the path of the trawl or scooped up in the nets. While the magnitude of these impacts are not the same for all bottom trawl fisheries, and depend on certain factors (e.g. the type of trawl gear used, habitat composition, life history of component species, the natural disturbance regime), even for those bottom trawls operating in the least sensitive benthic environments, these areas are often regularly trawled so have little chance to recover, and there are significant levels of bycatch. Fish that are too small or of the wrong species are thrown overboard, dead or dying. Demersal otter trawls commonly throw away over 30% of their catches (by weight) while beam trawls throw away up to 70% of their catches (by weight).

Considering, firstly, that management bodies have tended to put very few limits on where bottom trawls can operate, and secondly, that there is a serious lack of marine reserves to allow for both recovery and for scientific comparison of unfished and fished areas, Greenpeace does not currently support the use of bottom trawling.